1. Field of the Invention
The present invention relates to a semiconductor apparatus having a thin semiconductor film attached to a substrate, and more particularly to the electrical interconnections between the thin semiconductor film and the substrate. The invention also relates to image forming apparatus using the invented semiconductor device.
2. Description of the Related Art
An example of an image forming apparatus in which the invention can be applied is an electrophotographic printer employing light-emitting diodes as light sources. A conventional electrophotographic printer of this type has a light-emitting diode (LED) head comprising a linear array of LEDs paralleled by driver circuits that supply current to the LEDs. The LEDs are formed in one or more compound semiconductor chips comprising, for example, gallium arsenide (GaAs); the driver circuits are integrated into one or more silicon (Si) semiconductor chips. The LED array chips and the driver integrated circuit (IC) chips are interconnected by wire bonding, and have pads for the attachment of bonding wires.
The wire bonding pads are in general larger than the LEDs. If a separate wire bonding pad is provided for each LED, the layout of the wire bonding pads presents a problem to which various solutions have been adopted. One solution, for example, employs a staggered arrangement of wire bonding pads and offsets the wiring that leads from the wire bonding pads to the LEDs, as described in Japanese Unexamined Patent Application Publication No. 10-150221.
Regardless of how the wire bonding pads are laid out, they take up considerable space on each LED array chip. From the standpoint of effective use of expensive compound semiconductor materials, this is highly uneconomical, and it prevents the cost of the LED array chips from being reduced beyond a certain point. The wire bonding pads on the driver chips are similarly uneconomical.
Furthermore, although an LED array chip is typically about three hundred micrometers (300 μm) thick, the LEDs occupy only about the uppermost five micrometers (5 μm) of the chip thickness, the rest of the thickness being necessary merely for mechanical support. This also is uneconomical: only about 1/60 of the expensive compound semiconductor material is disposed at a depth shallow enough to be useful for emitting light.
A desirable solution to this problem would be to form the LEDs in a thin compound semiconductor film attached to the silicon substrate of the driver chips, and connect the LEDs to their driver circuits through conductive lines formed on the chip surface, instead of by wire bonding. Considerable compound semiconductor material could then be saved, since none would have to be used for mechanical support, or to provide space for wire bonding pads. Alignment of the electrical interconnections between the thin film and the substrate, however, then becomes a problem. If conductive lines run directly from the LEDs to the driver circuits, the thin semiconductor film needs to be positioned very accurately with respect to the output terminals of the driver circuits, making the combined LED and driver chip expensive to manufacture. Quality also becomes a problem, as the electrical characteristics of the combined LED/driver chip become very sensitive to slight alignment inaccuracies.